Caption: Photograph of a test tube containing DNA marked with a dye that fluoresces under ultraviolet light. The DNA is contained on plasmids taken from certain oral bacteria. Plasmids are structures which can exist and replicate independently of the chromosomes and which provide genetic instructions for certain cell activities, in this case the ability to produce acid from sugars. Dental caries is caused by the metabolism of the bacteria living in plaque attached to the surface of the tooth. Acid formed by the breakdown of dietary sugars gradually etches and decomposes the enamel of the tooth leading to decay.